Evidence for Suzuki-Miyaura cross-couplings catalyzed by ligated Pd3-clusters: from cradle to grave

Abstract

Pd_n clusters offer unique selectivity and exploitable reactivity in catalysis. Understanding the behavior of Pd_n clusters is thus critical for catalysis, applied synthetic organic chemistry and greener outcomes for precious Pd. The Pd₃ cluster, [Pd₃(μ-Cl)(μ-PPh₂)₂(PPh₃)₃][Cl] (denoted as Pd₃Cl₂), which exhibits distinctive reactivity, was synthesized and immobilized on a phosphine-functionalized polystyrene resin (denoted as immob-Pd₃Cl₂). The resultant material served as a tool to study closely the role of Pd₃ clusters in a prototypical Suzuki-Miyaura cross-coupling of 4-fluoro-1-bromobenzene and 4-methoxyphenyl boronic acid at varying low Pd ppm concentrations (24, 45, and 68 ppm). Advanced heterogeneity tests such as Hg poisoning and the three-phase test showed that leached mononuclear or nanoparticulate Pd are unlikely to be the major active catalyst species under the reaction conditions tested. EXAFS/XANES analysis from (pre)catalyst and filtered catalysts during and after catalysis has shown the intactness of the triangular structure of the Pd₃X₂ cluster, with exchange of chloride (X) by bromide during catalytic turnover of bromoarene substrate. This finding is further corroborated by treatment of immob-Pd₃Cl₂ after catalyzing the Suzuki-Miyaura reaction with excess PPh₃, which releases the cluster from the polymer support and so permits direct observation of [Pd₃(μ-Br)(μ-PPh₂)₂(PPh₃)₃]⁺ ions by ESI-MS. No evidence is seen for a proposed intermediate in which the bridging halogen on the Pd₃ motif is replaced by an aryl group from the organoboronic acid, i.e. formed by a transmetallation-first process. Our findings taken together indicate that the 'Pd₃X₂' motif is an active catalyst species, which is stabilized by being immobilized, providing a more robust Pd₃ cluster catalyst system. Non-immobilized Pd₃Cl₂ is less stable, as is followed by stepwise XAFS of the non-immobilized Pd₃Cl₂, which gradually changes to a species consistent with 'Pd_x(PPh₃)_y' type material. Our findings have far-reaching future implications for Pd₃ cluster involvement in catalysis, showing that immobilization of Pd₃ cluster species offers advantages for rigorous mechanistic examination and applied chemistries.